Effect of Al interlayers on growth and magneto-optic properties of MnBi thin films

Bi/Mn/Al/Bi/Mn multilayers with different Al-interlayer thickness have been deposited on glass substrates at room temperature. The multilayers are not protected by a passivating layer, such as SiOx, in order to investigate the interplay between Al-interlayer thickness and homogeneity, granularity, and topography before and after annealing. Due to the Al interlayer, the diffusion across the Bi/Mn/Al/Bi/Mn stack was reduced as demonstrated by x-ray analysis, scanning electron microscopy, and Rutherford backscattering spectrometry. During annealing, two (MnxBi100−x)100−yAly layers are formed which are separated by an Al interlayer. After annealing, polar Kerr hysteresis loops as measured from the front side of the films show a superposition of two hysteresis loops, opposite in sign, with different coercive fields. The presence of different coercive fields is explained by different granularity of the top and bottom (MnxBi100−x)100−yAly layer. The coercive field of the top (MnxBi100−x)100−yAly layer reaches values up to 1.25 T, while the bottom layer shows a constant value of 0.3 T independent of the Al-interlayer thickness. The opposite sign of the two contributions to the Kerr loops is explained by the different relative index of refraction at the air/(MnxBi100−x)100−yAly and the Al/(MnxBi100−x)100−yAly interface.